1. Field of the Invention
The present invention relates to an electro-photographic photosensitive member, and more particularly such photosensitive member adapted for use in an electro-photographic printer in which a laser beam performs line scanning according to image.
2. Related Background Art
Conventional electrophotographic printers utilizing laser beam scanning employ a gas laser of a relatively short wavelength such as helium-cadmium laser, argon laser or helium-neon laser, in combination with an electrophotographic photosensitive member involving a thick photosensitive layer such as a CdS-binder photosensitive layer or a charge transfer complex as disclosed in the IBM Journal of the Research and Development, Jan. 1971, p. 75-89. Consequently, the laser beam does not cause multiple reflection in the photosensitive layer, so that the formed image is practically free from interference fringe patterns.
However, the above-mentioned gas lasers are recently being replaced by semiconductor lasers for compactization and cost reduction of the apparatus. Since such semiconductor lasers generally have oscillation wavelengths in excess of 750 nm, there have been developed electrophotographic photosensitive members showing a high sensitivity in such long wavelength region.
Certain photosensitive members are already known to be sensitive to the light of a long wavelength region, for example in excess of 600 nm, such as a laminated photosensitive member provided with a photosensitive layer containing a phthalocyanine pigment such as copper-phthalocyanine or aluminum chloride-phthalocyanine, particularly a photosensitive layer of a laminate structure composed of a charge generation layer and a charge transport layer, or a photosensitive member utilizing a selenium-tellurium film.
However, such photosensitive member, sensitive to such long wavelength region, when exposed to a laser beam in an electrophotographic printer utilizing laser beam scanning, is incapable of satisfactory image reproduction due to the interference fringe patterns in the formed toner image.
This drawback is ascribed to the fact that the laser beam of a long wavelength is not completely absorbed in the photosensitive layer but is reflected by the substrate to generate multiple reflected lights which cause interference with the light reflected on the surface of the photosensitive layer.
In order to resolve such drawbacks there has been proposed to grain the surface of a conductive substrate employed in the photosensitive member by means of anodizing or sand blasting, or to provide a light absorbing layer or an anti-reflective layer between the photosensitive layer and the substrate, but it has not been possible, in practice, to completely eliminate the interference fringe pattern appearing at the image formation. Particularly the surface graining method can hardly attain uniform coarseness and tends to form relatively large grains in a certain proportion, and such large grains function as charge injection areas into the photosensitive layer, giving rise to white dots in the image formation (or black dots if a reversal development is employed). Besides it is difficult, in the manufacture, to produce conductive substrates of uniform coarseness within the same production lot.